feat: unified social identity layer — SocialIdentity, ports, use cases, adapter

SocialIdentity value object (Local|Remote), SocialCommand/SocialQuery
domain ports, 11 CQRS use cases w/ 14 tests, CompositeSocialAdapter
wrapping k_ap, 6 new domain events, handlers migrated from ap_service
to use cases. Closes #12 foundation — AP handlers + legacy cleanup TBD.

crates/domain/src/value_objects/social.rs

@@ -0,0 +1,17 @@
+use super::UserId;
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum SocialIdentity {
+    Local(UserId),
+    Remote { actor_url: String },
+}
+
+impl SocialIdentity {
+    pub fn is_local(&self) -> bool {
+        matches!(self, Self::Local(_))
+    }
+
+    pub fn is_remote(&self) -> bool {
+        matches!(self, Self::Remote { .. })
+    }
+}